This invention is concerned with devices for detecting a fluid analyte, of the type comprising a mass of porous material containing a reagent that reacts with the analyte to generate a signal, usually a change in color. Such devices may be divided into two classes. In one class, the analyte may be quantified by the rate of color change, or alternatively by the depth of color developed in a given time, the colour being generally uniform over the whole of the detection area. A device of this kind is described in U.S. Pat. No. 4,631,174 (Fuji) and comprises a laminate of a hydrophobic impervious layer with an entry aperture for liquid, a porous spreading layer, a chemical reagent layer (to change color by reaction with analyte in the test liquid), and a support layer. In such devices, access of the analyte to the chemical reagent layer is essentially rapid, and the rate of color generation is determined by the rate of reaction between the analyte and the reagent. A similar device is described in U.S. Pat. No. 4,772,560.
In the other class of devices, the analyte may be quantified by the rate of movement (or by the distance moved in a given time) of a moving boundary. Such a device comprises a mass of detection material, to which analyte gains access in a controlled manner comprising a region that has not been contacted by the analyte, and a region that has been contacted by the analyte, and a moving boundary between the two. The device of this invention is of the moving boundary type.
In environments where exposure to hazardous gases or vapors may occur, it is common practice to use a disposable gas monitor attached to the clothing. Typically these devices are used in industrial manufacture, mining, military and in fire and other emergency services. Length of stain tubes (e.g. sold under the Trademark Drager) are commonly used for this purpose and are commercially available for a wide variety of gases. In essence they consist of a glass tube containing a quantity of silica gel impregnated with chemicals that give a color change in the presence of the gas. When required, one end of the glass tube is snapped open in order to begin exposure of the reactive gel to the atmosphere. Exposure to the monitored gas produces a colored region that increases in length as a result of continued diffusion of the gas into the tube. The tubes are roughly calibrated (logarithmically) in order to give a concentration after one hour of exposure. The silica gel impregnated with reactive chemical, acts both as a diffusion medium and as a detection medium. Although very widely used, the tubes are complicated to manufacture, calibration may vary from tube to tube, and are expensive (for high volume manufacture) and awkward to use. In addition, they are subject to considerable variability in response due to dependence on the convective conditions prevailing.
Passive dosi-tubes are marketed under the Trademark Gastec. This device comprises a diffuser and a chemical reagent parallel in a glass tube whose end is broken open to admit the atmosphere. Silica gel impregnated with chemicals is used as the reagent. The tubes are generally similar to, and suffer from the same disadvantages as, Drager tubes.
U.S. Pat. No. 4,195,055 describes a time-temperature device of the moving-boundary type, but which has no inlet aperture. Also, the same material is used as a spreading medium and a detection medium.
British patent specification 2168480 describes a gas detector comprising a circular disc of detection material contained in a housing having an axial entry aperture whose size and length controls the rate of entry of the gas being detected and the movement of the moving boundary on the disc. There is a relatively large open space within the housing in front of the disc.
British patent specification 2084725 describes a gas detection device comprising a porous strip in a glass tube whose end is broken open to provide access to the atmosphere. Again, the strip is surrounded by a relatively large air gap. Again, the rate of movement of the moving boundary is controlled by the rate of entry of gas into the tube. The device as described in these two British patent specifications suffer from two disadvantages:
The rate of entry of gas into the device is controlled by conditions in the entry aperture, and these may be affected by atmospheric conditions, e.g. if the device is used in a wind or near a fan.
Once inside the device, the gas has relatively unimpeded flow to the detection layer, as a result of which the moving boundary may become blurred.